1. Field of the Invention
The invention relates to thickeners based on vinyl alcohol copolymers, and also to the use of these, in particular in compositions used in the building trades.
2. Background Art
Mixtures of lime hydrate and of cement are used for the masonry, rendering, troweling, bonding and restoration work carried out by the construction industry. Water-soluble polymers are added to the mixtures of lime hydrate and of cement to improve their workability and water-retention properties, with the intention of achieving very good workability and preventing the compositions of lime hydrate and of cement from losing water prior to setting on highly absorbent substrates. Premature loss of water leads to inadequate hardening or development of cracks in the construction material. The water-soluble polymers usually added are non-ionic cellulose ethers, such as methyl cellulose (MC), hydroxyethyl cellulose (HEC), methyl hydroxyethyl cellulose (MHEC), or methyl hydroxypropyl cellulose (MHPC).
For the purposes of the present invention, cellulose ethers are cellulose derivatives produced by the action of alkylating agents on cellulose in the presence of bases. Examples of alkylating agents are ethylene oxide, dimethyl sulfate, methyl chloride and methyl iodide. The only industrial method of preparing methyl celluloses is the reaction of alkali celluloses with methyl chloride, in the presence or absence of organic solvents, a process which generates toxicological concerns. The resultant cellulose ether therefore contains methanol, dimethyl ether, and relatively large amounts of sodium chloride as by-products. NaCl in particular can lead to corrosion problems in applications in the construction sector, and complicated purification processes therefore have to be used for its removal.
In applications in many other fields, cellulose ethers compete with entirely synthetic polymers, such as associative polyurethane thickeners, polyacrylates, polyamines, and polyamides, and also with naturally occurring water-soluble polymers, such as agar agar, tragacanth, carrageen, gum arabic, alginates, starch, gelatine, and casein. However, there have hitherto been no alternatives to cellulose ether for the workability and water-retention required for lime hydrate or cement-based compositions, in particular in cement-type systems with their high pH and high electrolyte content. A disadvantage of the cellulose ethers usually used in cement-type construction applications, in particular hydroxyethyl methyl cellulose, is that there is sometimes a considerable delay in cement setting. Although polyvinyl alcohols are known constituents of cement-type compositions, they are used only in a relatively low-molecular-weight form which cannot have any significant thickening effect. Although higher-molecular-weight polyvinyl alcohol polymers would be expected to have thickening properties, these polymers exhibit difficulties of low cold-water solubility and poor workability associated with this low solubility.
U.S. Pat. No. 5,565,027 discloses polyvinyl alcohols modified with hydroxyaldehydes used as cement plasticizers in cement-type systems. EP-A 272012 describes the use of vinyl alcohol copolymers as thickeners in aqueous systems such as emulsion paints, the copolymers containing not only vinyl alcohol units but also acrylic ester units having at least two ethylene oxide units in the ester radical. JP-A 10/087937 describes an improvement in the mechanical strength of cement-containing construction materials via addition of polyvinyl alcohol or of vinyl alcohol copolymers with defined solubility in aqueous Ca(OH)2 solution. The vinyl alcohol copolymers contain carboxyl units, sulfonate units and N-vinyl units.
JP-A 08/319395 describes the use of a composition made from alkali metal acetate salt and a vinyl alcohol copolymer having acetylacetone units as a thickener for polymer emulsions. JP-A 08/269132 describes fine-particle size, pulverulent vinyl alcohol homo- or copolymers with a defined proportion of syndiotactic, rather than the usual atactic, conformation, for improving the water-resistance of compositions modified with the same. JP-B 54-27382 describes a process for thickening copolymer dispersions based on carboxyl-functionality copolymers, where partially hydrolyzed polyvinyl alcohol is added. EP-A 458328 describes a thickener system intended for water-containing construction materials and composed of a combination of cellulose ether, polyvinyl alcohol and borax, its action being based on complex formation between polyvinyl alcohol and borax.
It was an object of the invention to provide an entirely synthetic water-soluble polymer which acts as a thickener in formulations used in civil engineering, and in particular in cement-type formulations, which produces formulations displaying excellent workability and mechanical properties, but which does not have the abovementioned disadvantages.
The invention provided thickeners based on fully or partially hydrolyzed vinyl alcohol copolymers with a molecular weight Mw greater than 100,000, obtainable by hydrolyzing vinyl acetate copolymers which, besides vinyl acetate units, also contain comonomer units which derive from one or more comonomers selected from the group consisting of 1-(C1-5)-alkylvinyl esters of C1-5 carboxylic acids, allyl esters, vinyl esters of alpha-branched C5-12 carboxylic acids having from 5 to 12 carbon atoms, and C1-18-alkyl (meth)acrylates, and in the form of their aqueous solution or in powder form. As is common in polymer chemistry, polymers may be described in terms of their constituent monomers without using language such as xe2x80x9cunits derived fromxe2x80x9d those monomers. Thus, a polymer derived from vinyl acetate monomer may be described as a vinyl acetate polymer even though once polymerized, the polymer will not contain vinyl acetate moieties.
The preferred 1-alkylvinyl ester is isopropenyl acetate. Preferred vinyl esters of alpha-branched carboxylic acids are those of alpha-branched carboxylic acids having from 9 to 11 carbon atoms, and particular preference is given to vinyl esters of alpha-branched carboxylic acids having 10 carbon atoms (VeoVa10, trade name of Shell). Preferred acrylic and methacrylic esters are those of C1-10 alcohols. Particular preference is given to methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, and 2-ethylhexyl methacrylate. From 0.05 to 2% by weight of auxiliary monomers, based on the total weight of the monomer mixture, may also be copolymerized. Examples of auxiliary monomers are ethylenically unsaturated mono- and dicarboxylic acids, preferably acrylic acid, methacrylic acid, fumaric acid, crotonic acid or maleic acid; ethylenically unsaturated carboxamides and ethylenically unsaturated carbonitriles, preferably acrylamide or acrylonitrile; cyclic carboxamides, such as N-vinylpyrrolidone and N-vinyl-xcex5-caprolactam, and ethylenically unsaturated carboxylic anhydrides, preferably maleic anhydride.
The degree of hydrolysis of the partially or fully hydrolyzed vinyl alcohol copolymers is generally from 75 to 100 mol %; in the case of xe2x80x9cfully hydrolyzedxe2x80x9d vinyl alcohol copolymers it is preferably from 97.5 to 100 mol %, more preferably from 98 to 99.5 mol %; and in the case of partially hydrolyzed vinyl alcohol copolymers it is preferably from 80 to 95 mol %, more preferably from 86 to 90 mol %. The proportion of comonomer units is from 0.1 to 50% by weight, preferably from 0.3 to 15% by weight, and most preferably from 0.5 to 6% by weight, based in each case on the total weight of the vinyl alcohol copolymer.
Particular preference is given to vinyl alcohol copolymers obtainable by hydrolyzing vinyl acetate copolymers having from 0.3 to 15% by weight of isopropenyl acetate, vinyl esters of alpha-branched C9-11 carboxylic acids, methyl, ethyl, butyl or 2-ethylhexyl acrylate, or 2-ethylhexyl methacrylate, or methyl methacrylate. Particular preference is also given to copolymers having from 0.3 to 15% by weight of isopropenyl acetate, units and from 0.3 to 15% by weight of units of vinyl esters of alpha-branched C9-11 carboxylic acids having from 9 to 11 carbon atoms. Finally, particular preference is also given to vinyl alcohol copolymers having from 0.5 to 6% by weight of isopropenyl acetate, from 0.5 to 6% by weight of vinyl esters of alpha-branched C10 carboxylic acids (VeoVa10), and from 0.5 to 6% by weight of methyl acrylate, and also to vinyl alcohol copolymers having from 0.5 to 6% by weight of isopropenyl acetate, from 0.5 to 6% by weight of 2-ethylhexyl methacrylate, and from 0.5 to 6% by weight of methyl acrylate.
The vinyl alcohol copolymers may be prepared by known processes, such as bulk, solution, suspension or emulsion polymerization. Solution polymerization preferably takes place in alcoholic solution, for example in methanol, ethanol or isopropanol. Suspension polymerization and emulsion polymerization are carried out in an aqueous medium. The polymerization is preferably carried out at a temperature of from 5 to 90xc2x0 C. with free-radical initiation by adding initiators usually used for the respective polymerization process. The vinyl alcohol units are introduced into the copolymer by copolymerization of vinyl acetate, the acetate radicals being hydrolyzed in a subsequent hydrolysis step in the same manner as other hydrolyzable monomer units. The molecular weight may be adjusted by adding regulators (i.e. chain transfer agents), by varying the solvent content, by varying the initiator concentration, by varying the temperature, or by combinations of the foregoing. After completion of the polymerization, solvent is removed by distillation where appropriate, or the polymer may be isolated from the aqueous phase by filtration.
The hydrolysis takes place in the conventional manner, under alkaline or acidic conditions established by addition of base or acid. The vinyl acetate copolymer to be hydrolyzed is preferably dissolved in alcohol such as methanol, at a solids content of from 5 to 50%. The hydrolysis is preferably carried out under basic conditions, for example by adding NaOH, KOH, or NaHCO3. The resultant vinyl alcohol copolymer may be isolated from the reaction mixture by filtration or distillation of the solvent mixture. The filtered product is then dried and ground by conventional methods.
It is also possible to obtain an aqueous solution by adding water, advantageously in the form of superheated steam, during the distillation of the organic solvents. For the work-up of an aqueous solution, preference is given to spray drying and to precipitation of the vinyl alcohol copolymer, for example using methanol. Work-up continues with a drying step and a grinding step. Grinding generally proceeds until the resultant average particle size is less than 1 mm, preferably less than 200 xcexcm.
The thickener may be used as an aqueous solution or in powder form, or as an additive in aqueous polymer dispersions or in water-redispersible polymer powders. It may be used alone or in admixture with other rheology additives. The amount of the thickener generally used is from 0.01 to 20% by weight of thickener composition (solid), based on the total weight of the composition to be thickened. The thickener is suitable for use as a thickener in any sector where rheological auxiliaries are used, for example as a thickener in cosmetics, in the pharmaceutical sector, in water-based silicone emulsions, in silicone oils, in coating compositions such as emulsion paints or textile coatings, as a thickener in adhesive compositions, or as a thickener in construction applications, either in hydraulically setting compositions or in non-hydraulically setting compositions, for example concrete, cement mortar, lime mortar, or gypsum mortar. There are other possible applications in water-containing mixes which also use cellulose ethers and starch ethers as thickeners. Particular preference is given to the construction applications. Very particular preference is given to cement-type construction applications, such as cement-type construction adhesives (tile adhesives), cement-type dry mortars, cement-type flowable compositions, cement-type renders, and cement-type exterior insulation system adhesives and cement-type non-shrink grouts.
Typical mixes for cement-type construction adhesives comprise from 5 to 80% by weight of cement, from 5 to 80% by weight of fillers such as quartz sand, calcium carbonate or talc, from 0.5 to 60% by weight of polymer dispersion or redispersible polymer powder, from 0.1 to 5% by weight of thickeners, and, where appropriate, other additives for improving stability, workability, open time, and water resistance. The data given here in % by weight are always based on 100% by weight of dry material of the mix and give a total of 100% by weight. The cement-containing construction adhesive mixes mentioned are used especially as tile adhesives for tiles of any type (earthenware, stoneware, porcelain, ceramics, natural tiles), indoors or outdoors, and are mixed with the appropriate amount of water prior to use.
The thickeners of the invention are also suitable for use in cement-free construction mixes, for example with the appropriate amount of gypsum or water glass as inorganic binder, and preferably in gypsum-containing compositions, such as gypsum renders and gypsum troweling compositions. The cement-free mixes are used especially in troweling compositions, tile adhesives, exterior insulation system adhesives, renders, and paints. Typical mixes for gypsum formulations comprise from 15 to 96% by weight of calcium sulfate, from 3 to 80% by weight of fillers, such as quartz sand, calcium carbonate or talc, from 0 to 5% by weight of hydrated lime, from 0 to 5% by weight of polymer dispersion or polymer powder, and also from 0.01 to 3% by weight of thickeners, and, where appropriate, other additives for improving stability, workability, open time and water resistance. The data in % by weight are always based on 100% by weight of dry material of the mix and give a total of 100% by weight.
In cement-type construction compositions, such as tile adhesives, the thickeners of the invention the are found to produce cement-type compositions of excellent workability and mechanical properties even without further rheological additives such as methyl hydroxyethyl cellulose. Until now, only partial replacement of cellulose ethers in cement-type construction formulations has been possible, for example by starch thickeners or water-soluble polyacrylates, but with considerable loss of technical quality in the construction composition. Using the thickener of the invention, up to 100% of the cellulose ethers can be replaced in cement-type applications without any losses in quality.
The examples below give further illustration of the invention.